Biggest fish have the greatest oxygen demand and often suffer first when O2 levels are lowThanks for that information. I didn't know that about the decaying process.....good to know!
I always have my air stone running, but as I said, those koi I lost were quite large. Massive in fact. So there very well could have been a decrease in oxygen.
I now have two air stones running and my net has been up for a while. I had it up before the leaves began to fall.
So, to the poster that was hesitant on installing a net....maybe you should rethink that idea.
All the years I've had nets on my pond, I've only had to rescue one songbird. Maybe keep a few birdbaths around your pond to help the birds out. Hopefully the birdbaths will thaw out once in a while with some direct sunlight.